Magnetorheological fluids that comprise suspensions of magnetic particles such as iron or iron alloys in a fluid medium are well known. The flow characteristics of these fluids can change by several orders of magnitude within milliseconds when subjected to a suitable magnetic field due to suspension of the particles. The ferromagnetic particles remain suspended under the influence of magnetic fields and applied forces. Such magnetorheological fluids have been found to have desirable electro-magnetomechanical interactive properties for advantageous use in a variety of magnetorheological (MR) damping devices, such as rotary devices including brakes and clutches, and linear-acting devices for damping linear motion or for providing controllable dissipative forces along the damper's axis.
In particular, linear acting MR dampers are commonly used in suspension systems, such as a vehicle suspension system and vehicle engine mounts. PCT patent application 10840, published Jan. 8, 1998 (the '840 application), discloses a conventional linear acting controllable vibration damper apparatus which includes a piston positioned in a magnetorheological fluid-filled chamber to form upper and lower chambers. The piston includes a coil assembly, a core, i.e. pole pieces, and an annular ring element positioned around the pole pieces to form an annular flow passage for permitting flow of the magnetorheological fluid between the chambers. When the piston is displaced, magnetorheological fluid is forced through the annular flow passage. When the coil is energized, a magnetic field permeates the channel and excites a transformation of the magnetorheological fluid to a state that exhibits damping forces.
In damper designs utilizing an annular flow passage, the radial width of the annular flow passage must be precisely set and maintained along the axial length of the passage throughout operation to ensure optimum, predictable control of the damping performance. Thus, the annular ring must be securely and concentrically mounted on the coil assembly to prevent axial and radial movement. The '840 application discloses the use of a plurality of bridge elements interconnecting the pole piece and the annular ring element. The bridge elements may include circumferentially spaced welds formed of nonmagnetic material. Also, each bridge may include a nonmagnetic pin to further locate and retain the pole relative to the ring. In another embodiment, the pole and ring are connected using a nonmagnetic plate positioned at one end of the assembly. The plate includes radially extending tabs forming bridging elements positioned outside and immediately adjacent the annular passage so as to extend across one end of the annular passage. The plate is secured to the pole piece and the ring by spot welds.
However, the means for connecting the ring and pole piece of the damper disclosed in the '840 application may result in specific disadvantages. For example, the plate tabs and welds are undesirably positioned immediately adjacent one end of the annular flow gap and, therefore, necessarily block fluid flow into the gap along the extent of the tabs and welds thereby disadvantageously reducing the effective shearing surface area of the damper resulting in a reduction in the MR effect. Also, the welds, pins and radial tabs of the plate each include blunt surfaces exposed to the fluid flow that undesirably impede the flow and increase uncontrollable drag forces which lead to a reduction in turn-up ratio performance of the assembly. In addition, the process required to position the plates and form the welds can be difficult and unnecessarily time consuming and expensive.
Also, in all MR damper designs, the various components of the piston assembly must be secured together and the piston rod must be effectively secured to the piston assembly. The '840 application discloses the use of a threaded connection between the piston rod and the piston core. In other designs, the rod extends completely through the core for connection with a nut which secure the entire assembly together. In both cases, an elongated central bore having an unnecessarily long length must be formed in the piston core to receive the rod and ensure complete passage of the rod or an effective threaded connection. Due to the reduction in core material, the magnetic flux generation capability of the core/pole pieces is significantly reduced or a larger core is required to achieve the same flux level. Also, the use of a nut attached to the outer end of a rod extending through the piston core adds to the length of the piston with additional parts resulting in an undesirably large and costly assembly possibly incapable of meeting the packaging constraints of a particular application.
Therefore, there is a need for a more compact, less costly MR damper capable of effectively and controllably damping motion.